Few studies have been undertaken on the conversion of benzene. One possible explanation is the presumption that benzene is stable over acid catalysts. Some early work disputed that art held presumption. Cf. Frilette, V. J., and Rubin, M. K. "Journal of Catalysis" 4, p. 310-311, 1965. Karge, H. G., and Ladebeck, J., "Studies in Surface Science and Catalysis", #5, Proceedings of the International Symposium, 1980.
Although in the past there has been little incentive for studying its conversion to other products because of the high value of benzene as a basic petrochemical, anticipation of environmental regulations in gasoline has provided the incentive.
Various benzene conversions have been proposed such as alkylation with olefins, alcohols, or olefinic fragments from paraffin cracking, and interaromatic conversions such as xylene transalkylation. The common feature of these benzene reduction schemes is the use of another reactant.
The conversion discussed herein is catalyzed by zeolites. Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are referred to as "molecular sieves" because the uniform pore size of a zeolite material may allow it to selectively absorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" auminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In the empirical formula, M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium; x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated M; and the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p. 5 (1974).
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5, its X-ray diffraction pattern, and its preparation are described in U.S. Pat. No. 3,702,886, the entire disclosure of which is incorporated by reference herein; zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and zeolite SZM-23 (U.S. Pat. No. 3,076,842), merely to name a few.
ZSM-11 is described in U.S. Pat. No. 3,709,979. That description, and in particular the X-ray diffraction pattern of said ZSM-11, is incorporated herein by reference.
ZSM-12 is described in U.S. Pat. No. 3,832,449. That description, and in particular the X-ray diffraction pattern disclosed therein, is incorporated herein by reference.
ZSM-22 is described in U.S. patent application Ser. No. 373,451 filed Apr. 30, 1982, and now pending. The entire description thereof is incorporated herein by reference.
ZSM-23 is described in U.S. Pat. No. 4,076,842. The entire content thereof, particularly the specification of the X-ray diffraction pattern of the disclosed zeolite, is incorporated herein by reference.
ZSM-35 is described in U.S. Pat. No. 4,016,245. The description of that zeolite, and particularly the X-ray diffraction pattern thereof, is incorporated herein by reference.
ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859. The description of that zeolite, and particularly the specified X-ray diffraction pattern thereof, is incorporated herein by reference.
ZSM-57 is a zeolite, the X-ray diffraction pattern and synthesis of which are described in EP 0,174,121.
It is to be understood that by incorporating by reference the foregoing patents and patent applications to describe examples of specific members of the novel class with greater particularity, it is intended that identification of the therein disclosed crystalline zeolites by resolved on tha basis of their respective X-ray diffraction patterns. It is the crystal structure, as identified by the X-ray diffraction "fingerprint", which establishes the identity of the specific crystalline zeolite material. The crystal structure of known zeolites may include gallium, boron, iron and chromium as framework elements, without changing its identification by the X-ray diffraction "fingerprint"; and these gallium, boron, iron and chromium containing silicates and aluminosilicates may be useful, or even preferred, in some applications described herein.
The members of the class of zeolites useful herein have an effective pore size of generally from about 5 to about 8 Angstroms, such as to freely sorb normal hexane. In addition, the structure must provide constrained access to larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings of silicon and aluminum atoms, then access by molecules of larger cross-section than normal hexane is excluded and the zeolite is not of the desired type. Windows of 10-membered rings are preferred, although, in some instances, excessive puckering of the rings or pore blockage may render these zeolite ineffective.
Although 12-membered rings in theory would not offer sufficient constraint to produce advantageous conversions, it is noted that the puckered 12-ring structure of TMA offretite does show some constrained access. Other 12-ring structures may exist which may be operative for other reasons, and therefore, it is not the present intention to entirely judge the usefulness of the particular zeolite solely from theoretical structural considerations.
A convenient measure of the extent to which a zeolite provides control to molecules of varying sizes to its internal structure is the Constraint Index of the zeolite. Zeolites which provide a highly restricted access to and egress from its internal structure have a high value for the Constraint Index, and zeolites of this kind usually have pores of small size, e.g. less than 5 Angstroms. On the other hand, zeolites which provide relatively free access to the internal zeolite structure have a low value for the Constraint Index, and usually pores of large size, e.g. greater than 8 Angstroms. The method by which Constraint Index is determined is described fully in U.S. Pat. No. 4,016,218, incorporated herein by reference for details of the method.
______________________________________ CI (at test temperature) ______________________________________ ZSM-4 0.5 (316.degree. C.) ZSM-5 6-8.3 (371.degree. C.-316.degree. C.) ZSM-11 5-8.7 (371.degree. C.-316.degree. C.) ZSM-12 2.3 (316.degree. C.) ZSM-20 0.5 (371.degree. C.) ZSM-22 7.3 (427.degree. C.) ZSM-23 9.1 (427.degree. C.) ZSM-34 50 (371.degree. C.) ZSM-35 4.5 (454.degree. C.) ZSM-48 3.5 (538.degree. C.) ZSM-50 2.1 (427.degree. C.) TMA Offretite 3.7 (316.degree. C.) TEA Mordenite 0.4 (316.degree. C.) Clinoptilolite 3.4 (510.degree. C.) Mordenite 0.5 (316.degree. C.) REY 0.4 (316.degree. C.) Amorphous Silica-alumina 0.6 (538.degree. C.) Dealuminized Y 0.5 (510.degree. C.) Erionite 38 (316.degree. C.) Zeolite Beta 0.6-2.0 (316.degree. C.-399.degree. C.) ______________________________________
The above-described Constraint Index is an important and even critical definition of those zeolites which are useful in the instant invention. The very nature of this parameter and the recited technique by which it is determined, however, admit of the possibility that a given zeolite can be tested under somewhat different conditions and thereby exhibit different Constraint Indices. Constraint Index seems to vary somewhat with severity of operations (conversion) and the presence or absence of binders. Likewise, other variables, such as crystal size of the zeolite, the presence of occluded contaminants, etc., may affect the Constraint Index. Therefore, it will be appreciated that it may be possible to so select test conditions, e.g. temperature, as to establish more than one value for the Constraint Index of a particular zeolite. This explains the range of Constraint Indices for some zeolites, such as ZSM-5, ZSM-11 and Beta.
It is to be realized that the above CI values typically characterize the specified zeolites, but that such are the cumulative result of several variables useful in the determination and calculation thereof. Thus, for a given zeolite exhibiting a CI value within the range of 1 to 12, depending on the temperature employed during the the test method within the range of 290.degree. C. to about 538.degree. C., with accompanying conversion between 10% and 60%, the CI may vary within the indicated range of 1 to 12. Likewise, other variables such as the crystal size of the zeolite, the presence of possibly occluded contaminants and binders intimately combined with the zeolite may affect the CI. It will accordingly be understood to those skilled in the art that the CI, as utilized herein, while affording a highly useful means for characterizing the zeolites of interest is approximate, taking into consideration the manner of its determination, with the possibility, in some instances, of compounding variable extremes. However, in all instances, at a temperature within the above-specified range of 290.degree. C. to about 538.degree. C., the CI will have a value for any given zeolite of interest herein within the approximate range of 1 to 12.